U.S. patent application number 16/374925 was filed with the patent office on 2019-08-01 for two-terminal spintronic devices.
The applicant listed for this patent is Regents of the University of Minnesota. Invention is credited to Mahdi Jamali, Yang Lv, Jian-Ping Wang.
Application Number | 20190237510 16/374925 |
Document ID | / |
Family ID | 62489646 |
Filed Date | 2019-08-01 |
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United States Patent
Application |
20190237510 |
Kind Code |
A1 |
Wang; Jian-Ping ; et
al. |
August 1, 2019 |
TWO-TERMINAL SPINTRONIC DEVICES
Abstract
This disclosure describes an example device that includes a
first contact line, a second contact line, a spin-orbital coupling
channel, and a magnet. The spin-orbital coupling channel is coupled
to, and is positioned between, the first contact line and second
contact line. The magnet is coupled to the spin-orbital coupling
channel and positioned between the first contact line and the
second contact line. A resistance of the magnet and spin-orbital
coupling channel is a unidirectional magnetoresistance.
Inventors: |
Wang; Jian-Ping; (Shoreview,
MN) ; Lv; Yang; (New Brighton, MN) ; Jamali;
Mahdi; (Folsom, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Regents of the University of Minnesota |
Minneapolis |
MN |
US |
|
|
Family ID: |
62489646 |
Appl. No.: |
16/374925 |
Filed: |
April 4, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15839081 |
Dec 12, 2017 |
10283561 |
|
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16374925 |
|
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62434166 |
Dec 14, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03K 19/18 20130101;
H01L 43/10 20130101; G11C 11/1673 20130101; H01L 27/222 20130101;
H03K 19/177 20130101; H01L 43/08 20130101; G11C 11/1659 20130101;
H01L 27/224 20130101; G11C 11/1675 20130101; H03K 19/1776 20130101;
G11C 11/16 20130101; G11C 11/161 20130101; H01L 43/04 20130101;
G11C 11/18 20130101; H01L 27/22 20130101 |
International
Class: |
H01L 27/22 20060101
H01L027/22; G11C 11/18 20060101 G11C011/18; H03K 19/177 20060101
H03K019/177; H03K 19/18 20060101 H03K019/18; G11C 11/16 20060101
G11C011/16; H01L 43/10 20060101 H01L043/10; H01L 43/08 20060101
H01L043/08; H01L 43/04 20060101 H01L043/04 |
Goverment Interests
GOVERNMENT INTEREST
[0002] This invention was made with government support under Grant
No. HR0011-13-3-0002 awarded by Department of Defense/Defense
Advanced Research Projects Agency (DARPA). The government has
certain rights in the invention.
Claims
1. A method of manufacturing a memory device comprising: forming a
first contact line; forming a second contact line; forming a
spin-orbital coupling channel such that the spin-orbital coupling
channel is coupled to, and is positioned between, the first contact
line and second contact line, and such that the spin-orbital
coupling channel is directly coupled to at least one of the first
contact line or the second contact line; and forming a magnet such
that the magnet is positioned between the first contact line and
the second contact line and such that the magnet is directly
coupled to the spin-orbital coupling channel and the at least one
of the first contact line or the second contact line, wherein
forming the spin-orbital coupling channel comprises forming the
spin-orbital coupling channel such that a resistance of the magnet
and spin-orbital coupling channel is a unidirectional
magnetoresistance and such that the unidirectional resistance is
not based on the magnetization direction of the magnet relative to
a magnetization direction of another magnet.
2. The method of claim 1, wherein the unidirectional
magnetoresistance of the magnet and spin-orbital coupling channel
comprises a first resistance when a magnetization direction of the
magnet is a first direction and a second resistance when the
magnetization direction of the magnet is a second direction.
3. The method of claim 2, wherein the first direction is
approximately 0 degrees and the second direction is approximately
180 degrees.
4. The method of claim 1, wherein forming the magnet comprises
arranging the magnet vertically relative to the the spin-orbital
coupling channel.
5. The method of claim 1, wherein a first interface between the
spin-orbital coupling channel and the magnet defines a first plane,
wherein the first plane is substantially perpendicular to a second
plane defined by a second interface between the magnet and the
first contact line or a third plane defined by a third interface
between the spin-orbital coupling channel and the first contact
line.
6. The method of claim 1, wherein forming the magnet comprises
forming the magnet such that the magnet is directly coupled to the
first contact line and the second contact line, and wherein forming
the spin-orbital coupling channel comprises forming the
spin-orbital coupling channel such that the spin-orbital coupling
channel is directly coupled to the first contact line and the
second contact line.
7. The method of claim 1, further comprising forming a selector
that is directly coupled to the magnet, the spin-orbital channel,
and at least one of the first contact line or the second contact
line.
8. The method of claim 1, further comprising: forming a via coupled
to the first contact line; and forming a selector line coupled to
the spin-orbital coupling channel and the via.
9. The method of claim 1, wherein forming the spin-orbital coupling
channel comprises forming the spin-orbital channel from one or more
of: a heavy metal, or a topological insulator.
10. The method of claim 1, wherein the spin-orbital coupling
channel comprises a first spin-orbital coupling channel, the magnet
comprises a first magnet, the method further comprising: forming a
third contact line; forming a second spin-orbital coupling channel
such that the second spin-orbital coupling channel is coupled to,
and is positioned between, the second contact line and the third
contact line; and forming a second a magnet such that the second
magnet is coupled to the second spin-orbital coupling channel and
is positioned between the second contact line and the third contact
line, wherein forming the second spin-orbital coupling channel
comprises forming the second spin-orbital coupling channel such
that a resistance of the second magnet and the second spin-orbital
coupling channel is a unidirectional magnetoresistance.
11. The method of claim 1, wherein the spin-orbital coupling
channel comprises a first spin-orbital coupling channel and the
magnet comprises a first magnet, the method further comprising:
forming a plurality of spin-orbital coupling channels and a
corresponding plurality of magnets sandwiched between the first
contact line and the second contact line such that a resistance of
a respective spin-orbital coupling channel of the plurality of
spin-orbital coupling channels and the corresponding magnet of the
plurality of magnets is a unidirectional magnetoresistance.
12. A method of manufacturing a memory device, the method
comprising: forming a first contact line; forming a second contact
line; forming a third contact line; forming a first spin-orbital
coupling channel such that the first spin-orbital coupling channel
is positioned between the first contact line and second contact
line and is directly coupled to at least one of the first contact
line or the second contact line; forming a second spin-orbital
coupling channel such that the second spin-orbital coupling channel
is positioned between the second contact line and the third contact
line and is directly coupled to at least one of the second contact
line or the third contact line; forming a first magnet such that
the first magnet is positioned between the first contact line and
the second contact line and is directly coupled to the first
spin-orbital coupling channel; and forming a second magnet such
that the second magnet is positioned between the second contact
line and the third contact line and is directly coupled to the
second spin-orbital coupling channel, wherein forming the first
spin-orbital coupling channel comprises forming the first
spin-orbital coupling channel such that a resistance of the first
magnet and the first spin-orbital coupling channel is a first
unidirectional magnetoresistance, and wherein forming the second
spin-orbital coupling channel comprises forming the second
spin-orbital coupling channel such that a resistance of the second
magnet and the second spin-orbital coupling channel is a second
unidirectional magnetoresistance.
13. The method of claim 12, wherein forming the first magnet
comprises forming the first magnet such that the first magnet is
directly coupled to the at least one of the first contact line or
the second contact line, and wherein the forming the second magnet
comprises forming the second magnet such that the second magnet is
directly coupled to the at least one of the second contact line or
the third contact line.
14. The method of claim 12, further comprising: forming a first
selector such that the first selector is directly coupled to the
first magnet, the first spin-orbital channel, and at least one of
the first contact line or the second contact line, and forming a
second selector such that the second selector is directly coupled
to the second magnet, the second spin-orbital channel, and at least
one of the second contact line or the third contact line.
15. The method of claim 12, wherein forming the first magnet and
the first spin-orbital coupling channel comprises forming the first
magnet and the first spin-orbital coupling channel such that the
first magnet and the first spin-orbital coupling channel are
arranged in a crossbar configuration with the second magnet and
second spin-orbital coupling channel.
16. The method of claim 12, wherein forming the first magnet and
the first spin-orbital coupling channel comprises forming the first
magnet and the first spin-orbital coupling channel such that the
first magnet and first spin-orbital coupling channel are arranged
on top of the second magnet and second spin-orbital coupling
channel to form a 3D memory architecture.
17. A method comprising: outputting, by a controller, a write
current through a spin-orbital coupling channel of a memory device
that includes a first contact line coupled to a first side of the
spin-orbital coupling channel, a second contact line coupled to a
second side of the spin-orbital coupling channel that is opposite
the first side, and a magnet coupled to the spin-orbital coupling
channel and positioned between the first contact line and the
second contact line, to set a resistance of the magnet of the
memory device to a first resistance level indicative of a first
digital value or a second resistance level indicative of a second
digital value; and outputting, by the controller, a read current
through the spin-orbital coupling channel to determine whether a
unidirectional magnetoresistance of the magnet and the spin-orbital
coupling channel is at the first resistance level or the second
resistance level, without outputting a current through the
magnet.
18. The method of claim 17, wherein outputting the write current
comprises outputting the write current with a first current density
greater than or equal to a threshold current density for setting a
magnetization direction of the magnet, and wherein outputting the
read current comprises outputting the read current with a second
current density less than the threshold current density.
Description
[0001] This application is a continuation of U.S. application Ser.
No. 15/839,081, filed Dec. 12, 2017, which claims the benefit of
U.S. Provisional Application No. 62/434,166, filed Dec. 14, 2016,
the entire contents of which are incorporated by reference
herein.
TECHNICAL FIELD
[0003] This disclosure relates to articles including magnetic
structures, and more particularly, magnetic structures for memory
and logic devices.
BACKGROUND
[0004] The scaling of conventional semiconductor devices may be
limited by factors including device reliability and increased power
consumption. Improvement in the performance of memory and
computational devices is continuously pursued. Spin-based or
spintronic devices may be used as alternatives to or in conjunction
with electronic devices. Spin-based effects may be used by devices
such as spintronic devices that harness the intrinsic spin of
electrons and their associated magnetic moments, in addition to
electronic phenomena that arise from the fundamental electronic
charges of electrons. Magnetic structures may be used in spintronic
devices including memory and computational devices. For example,
memory devices such as magnetic random access memory (MRAM) or
spin-transfer torque random access memory (STT-RAM) may be based on
the relative magnetic orientation of multiple magnetic layers.
SUMMARY
[0005] In general, the disclosure describes examples of
two-terminal spintronic devices such as memory or logic devices.
The two-terminal spintronic devices may be based on a
unidirectional resistance of a nonmagnetic material (e.g., a
spin-orbital coupling channel) and a ferromagnetic material (e.g.,
a magnet), which may be referred to as unidirectional spin Hall
magnetoresistance (USMR) effects. For USMR effect, current of a
threshold amplitude through a non-magnetic (NM) material that has
strong spin-orbit interaction (SOI) causes a magnetization
direction of a ferromagnetic (FM) layer to switch. The
magnetization direction of the FM layer may be correlated with the
resistance at the interface between the FM material and the NM
material. In some examples, the resistance of the FM material and
the NM material may be constant. In other words, the resistance of
the FM material alone and the resistance of the NM material alone
may not change, but the total resistance of the combination of the
FM material and the NM material may be a unidirectional
magnetoresistance that changes depending on the magnetization
direction of the FM layer (e.g., the resistance at the interface
between the FM material and the NM material may change, which may
cause a change in the total resistance). By applying a current
greater than the threshold current density (e.g., amplitude, but
not limited to amplitude) through the NM material, a controller
circuit may set the resistance of the combined NM material and FM
material (e.g., the current may change the resistance at the
interface). Then, by applying a current less than the threshold
current density through the NM material, the controller circuit may
read a voltage across the combination of the NM material, the FM
material, and the interface between the NM material and the FM
material, such that the controller circuit may determine the
resistance of the combination of the NM material, the FM material,
and the interface. In this way, the resistance of the device may be
used as way to convey a digital high or a digital low.
[0006] In some examples, the disclosure describes a device that
includes a first contact line, a second contact line, a
spin-orbital coupling channel, and a magnet. The spin-orbital
coupling channel is coupled to, and is positioned between, the
first contact line and second contact line. The magnet is coupled
to the spin-orbital coupling channel and positioned between the
first contact line and the second contact line. A resistance of the
magnet and spin-orbital coupling channel is a unidirectional
magnetoresistance. The magnet is directly coupled to the
spin-orbital coupling channel and at least one of the first contact
line or the second contact line. The spin-orbital coupling channel
is directly coupled to the at least one of the first contact line
or the second contact line.
[0007] In another example, the disclosure describes a device that
includes a first contact line, a second contact line, a third
contact line, a first spin-orbital coupling channel, a second
spin-orbital coupling channel, a first magnet, and a second magnet.
The first spin-orbital coupling channel is positioned between the
first contact line and second contact line and directly coupled to
at least one of the first contact line or the second contact line.
The second spin-orbital coupling channel is positioned between the
second contact line and the third contact line and directly coupled
to at least one of the second contact line or the third contact
line. The first magnet is positioned between the first contact line
and the second contact line and directly coupled to the first
spin-orbital coupling channel. The second magnet is positioned
between the second contact line and the third contact line and
directly coupled to the second spin-orbital coupling channel. A
resistance of the first magnet and the first spin-orbital coupling
channel is a unidirectional magnetoresistance and a resistance of
the second magnet and second spin-orbital coupling channel is a
unidirectional magnetoresistance.
[0008] In yet another example, the disclosure describes a device
that includes a first contact line, a second contact line, a
spin-orbital coupling channel, a magnet, and a controller circuit.
The spin-orbital coupling channel is configured to receive a read
current and a write current. The spin-orbital coupling channel is
coupled to, and is positioned between, the first contact line and
second contact line. The magnet is coupled to the spin-orbital
coupling channel and positioned between the first contact line and
the second contact line. A resistance of the magnet and
spin-orbital coupling channel is a unidirectional
magnetoresistance. The controller circuit is configured to: output
a write current through the spin-orbital coupling channel to set a
resistance of the magnet to a first resistance level indicative of
a first digital value or a second resistance level indicative of a
second digital value; and output the read current through the
spin-orbital coupling channel to determine whether the resistance
is at the first resistance level or the second resistance level,
without outputting a current through the magnet.
[0009] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0010] FIG. 1 is a conceptual diagram illustrating an example of
spin Hall effect.
[0011] FIG. 2A is a block diagram illustrating an example of a
three-terminal spintronic device.
[0012] FIG. 2B is a graph illustrating a change in resistance of
the device of FIG. 2A as a function of current.
[0013] FIGS. 3A and 3B are conceptual diagrams illustrating
unidirectional spin Hall magnetoresistance (USMR) effects.
[0014] FIGS. 4A and 4B are conceptual diagrams illustrating writing
to and reading from, respectively, a two-terminal spintronic
device.
[0015] FIGS. 5A-5C are conceptual diagrams illustrating examples of
crossbar and 3D architectures for memory devices using two-terminal
spintronic devices.
[0016] FIGS. 6A-6D are conceptual diagrams illustrating examples of
memory cell layouts.
[0017] FIGS. 7A and 7B are conceptual diagrams illustrating
examples of cross bar memory architecture.
[0018] FIG. 8A is a conceptual diagram illustrating an example of
nanowire cell.
[0019] FIG. 8B is a conceptual diagram illustrating an example of a
nanowire cell in cross bar.
[0020] FIGS. 9A an 9B are conceptual diagrams illustrating an
example for sensing magnetic nano-particles.
[0021] FIG. 9C is a graph illustrating example threshold for
sensing presence of magnetic nano-particles.
[0022] FIG. 10A is a conceptual diagram illustrating resistance
measurement setup and definitions of rotation planes.
[0023] FIGS. 10B and 10C are graphs illustrating resistance as a
function of angle.
[0024] FIG. 11A is a conceptual diagram illustrating
transverse/Hall resistance measurement setup.
[0025] FIGS. 11B and 11C are graphs illustrating resistance as a
function of angle and external fields, respectively.
[0026] FIGS. 12A and 12B are graphs illustrating contribution for
total resistance for different examples of structures that exhibit
USMR effects.
[0027] FIGS. 13A and 13B are graphs illustrating USMR per current
density per total resistance and sheet USMR per current density,
respectively, as a function of temperature.
[0028] FIG. 14 is a graph illustrating resistance as a function of
magnetic field for different structures.
[0029] FIGS. 15A-15C are graphs illustrating resistance as a
function of magnetic field for additional structures are different
temperatures.
[0030] FIG. 16 is a graph illustrating sheet USMR comparison.
[0031] FIG. 17 is a flowchart illustrating example operations of a
device configured to write to and read from a two-terminal
spintronic device.
DETAILED DESCRIPTION
[0032] This disclosure describes a two-terminal spintronic device,
which may overcome limitations of three-terminal spintronic devices
for memory and computation applications. The example techniques may
be applicable to support a magnetic cross-bar memory architecture,
a magnetic 3D memory architecture, and computation in magnetic
memory architecture.
[0033] Current semiconductor devices face many challenges and
bottlenecks including the difficulty to further scale, increased
dynamic and static power consumption, and limitations of speed.
Instead of using the charge of electrons to represent, store,
transfer and compute information, the spin momentum of electrons
can also be used. The technology of utilizing electron spins is
called Spintronics.
[0034] Spintronics may feature better scalability, better speed,
less power consumption, and non-volatility. Example uses of
spintronics such as in spin transfer torque random access memory
(STT-RAM) have already emerged. However, devices like STT-RAM may
still exhibit issues such as lower switching efficiency, which in
turn leads to worse power consumption and worse reliability. The
spin Hall effect in some heavy metals and the topological
insulators may be alternatives to achieve magnetization switching
more efficiently than STT.
[0035] The spin Hall effect (SHE) is a phenomenon of electron spins
to deflect transversely when a charge current is applied in a
non-magnetic (NM) material that has strong spin-orbit interaction
(SOI). This may lead to polarized spins flow, which is called spin
current, and spin accumulation at interfaces. If a ferromagnetic
(FM) layer is in contact with the NM layer, the spins generated by
the SHE will interact with the magnetization of the FM due to the
transfer of angular momentum from the spin to the magnetization. In
some cases, with the transfer of the angular momentum, the
magnetization in FM can be switched by utilizing the SHE.
[0036] Topological insulators (TIs) are a kind of material whose
bulk is electrically insulating but whose surfaces may be
conductive. The electrons flowing on the surface of a TI may be
spin polarized (e.g., completely spin polarized) due to the
spin-momentum locking of the surface states. These electron spins
may exert large torques on magnetization nearby. Although TIs
possess different physics compared to heavy metals, TIs may
generate spins and switch magnetizations in a similar way in terms
of the device structure, directions of current and torque.
Accordingly, both heavy metals and TIs may be considered as
functioning in a SHE or SHE-like fashion when used to switch a
magnet.
[0037] However, while such devices can be made more efficient to
switch using the SHE or TIs, the device may require a third
terminal and a magnetic tunneling junction (MTJ) structure in order
to read out the magnetization state. This makes such devices more
difficult to fabricate and weakens their scalability. Stated
another way, SHE or TIs may be used to set the magnetization state
of a magnet to a first magnetization state or a second
magnetization state. The first magnetization state corresponds to a
first digital value (e.g., logic high or low), and the second
magnetization state corresponds to a second digital value (e.g.,
other of the logic high or low). As described above, the
magnetization state is set by driving a current through the NM
material. In this case, the input of the NM material and the output
of the NM material form first and second terminals, respectively.
In this way, SHE or TIs are used to store a digital value on the
magnet.
[0038] However, reading the digital value, by determining the
magnetization state of the magnet, may require another terminal and
an MTJ structure that includes a free FM layer and a fixed FM
layer. Conventionally, the resistance of a single magnetic layer in
a first magnetization state is the same as the resistance of the
single magnetic layer in a second magnetization state. Thus, in
order to store and subsequently read the digital value, a
conventional memory or logic device that utilizes the SHE includes
a MTJ structure with a free FM layer and a fixed FM layer because
the resistance of the MTJ as a whole changes depending on whether
the free FM layer is parallel or anti-parallel to the fixed FM
layer. Thus, when the device includes an MTJ, a current is output
through the MTJ (which forms the third terminal) to determine the
resistance of the MTJ and hence the digital value stored by the
memory device.
[0039] This disclosure describes examples where a two-terminal
spintronic device may be used, rather than the three-terminal
example described above. The use of such two-terminal devices
reduces needed components, which allows for the spintronic devices
to be scaled down even further. Such reduction in size and
reduction in terminals may be beneficial for examples of memories
and logic devices.
[0040] For example, devices that exhibit a unidirectional
resistance of a spin-orbital coupling channel (e.g., a spin hall
channel) and a magnet, which may be referred to as unidirectional
spin Hall magnetoresistance (USMR), may provide for devices with
two terminals that do not require an MTJ for reading the
magnetization state of the magnet. In some examples, the
unidirectional magnetoresistance of the spin-orbital coupling
channel may change from a first resistance value to a second
resistance value depending on the magnetization direction of the
magnet (e.g., the magnetization direction of the magnet may affect
the resistance of an interface between the magnet and the
spin-orbital coupling channel).
[0041] According to techniques of this disclosure, unidirectional
resistance means the resistance of the spin-orbital channel and the
magnet is a first resistance level when the magnetization direction
of the magnet is a first direction, and the resistance is a second
resistance level when the magnetization direction is a second
direction. For instance, the unidirectional magnetoresistance may
be a first value when the magnetization direction of the magnet is
at approximately 0 degrees (e.g., relative to a reference
direction, which may be defined by the direction of the electron
spin of the spin-orbital coupling channel) and a second value when
the magnetization direction of the magnet is approximately 180
degrees. In some examples, the resistance values may be different
at magnetization directions other than 0 degrees and 180 degrees
depending on the structure of the magnet. For example, the first
magnetization direction may be approximately 45 degrees (e.g.,
.+-.44.99 degrees) and the second magnetization direction may be
approximately 135 degrees (e.g., .+-.44.99 degrees). As another
example, the first magnetization direction may be approximately
0.degree. (e.g., .+-.up to 89.99.degree.) and the second
magnetization direction may be approximately 180.degree. (e.g.,
.+-.up to 89.99.degree.). In other examples, the first
magnetization direction may be approximately 0.degree. (e.g.,
.+-.45.degree.) and the second magnetization direction may be
approximately 180.degree. (e.g., .+-.45.degree.). As yet another
example, the first magnetization direction may be approximately
30.degree. (e.g., .+-.59.99.degree.) and the second magnetization
direction may be approximately 150.degree. (e.g.,
.+-.59.99.degree.). In some examples, the magnetization directions
may be defined in the 180.degree. to 360.degree. range.
[0042] In some examples, the resistance may be different for more
than two magnetization states. In other words, the resistance may
be a first value when the magnetization direction is approximately
0 degrees, a second value when the magnetization direction is
approximately 90 degrees, and a third value when the magnetization
direction is approximately 180 degrees.
[0043] In a simple FM/NM structure, due to the spins accumulation
from SHE at the interface, the USMR is present. The unique angular
dependency of USMR, which is also left-right sensitive like the
tunneling magnetoresistance, makes it a substitute for three
terminal devices. Thus, with USMR, a simple device made out of
NM/FM bilayer may be capable of both writing and reading without
any additional (third) terminal or MTJ structure. USMR may also be
present in TI/FM systems, thus making the example techniques
applicable in the same way for TI/FM devices.
[0044] This disclosure describes examples of combining the
spin-orbit torque switching by SHE or TIs as writing mechanism with
USMR as a reading mechanism for a simple, yet potentially powerful
design of a memory/logic device featuring only two terminals. This
allows the use of more efficient spin-orbit torque (SOT) switching
while still keeping the device at minimal two terminals so that it
can be easily embedded into mature crossbar memory architectures,
with or without selectors, which are used, for example, with MTJs
in STT-RAM.
[0045] As described above, the unidirectional resistance refers to
the combined resistance of the magnet and the spin-orbital coupling
channel. The magnetization direction of the magnet sets the
resistance of the combination of the magnet and the spin-orbital
coupling channel (e.g., by setting the interface of magnet and the
spin-orbital coupling channel), and therefore, the resistance is
not based on the magnetization direction of the magnet relative to
a magnetization direction of another layer (sometimes called fixed
layer for MTJs).
[0046] FIG. 1 is a conceptual diagram illustrating an example of
spin Hall effect. In the illustrated example, a charge current (Jc)
is passed through a spin-orbital coupling channel 102. In some
examples, spin-orbital coupling channel 102 may be a spin Hall
channel, a spin channel that provides a Rashba effect with a
magnet, or a channel that provides a unidirectional interface
effect to induce the USMR effect. Due to the spin orbit
interactions (SOI), electrons 114 of different spin directions are
deflected in directions that are at right angles to their spins. As
illustrated in FIG. 1, electrons deflected up/down carry spins
pointing to right/left, respectively. At the surface/interface of
the spin-orbital coupling channel 102, these spin-polarized
electrons accumulate, which is referred to as spin accumulation.
The accumulated spins of electrons 102 can potentially exert a
torque on magnet 104 if the magnet 104 is in close contact to a
surface of the spin-orbital coupling channel 102. The torque may
switch the magnetization state (e.g., magnetization direction) of
the magnet 104.
[0047] FIG. 2A is a block diagram illustrating an example of a
three-terminal spintronic device. FIG. 2A illustrates an MTJ 204
that includes a CoFeB fixed layer, a MgO insulation layer, and a
CoFeB free layer. The magnetization direction (e.g., magnetization
state) of the MTJ 204, and particularly the CoFeB free layer,
changes based on the flow of the current through the spin-orbital
coupling channel 202. As illustrated, spin-orbital coupling channel
202 is formed by Ta. Other example materials for MTJ 204 and
spin-orbital coupling channel 202 are possible. Spin Hall effect
can completely switch the magnetization of the free layer of an
in-plane magnetic layer (e.g., CoFeB free layer of MTJ 204).
[0048] FIG. 2B is a graph illustrating a change in resistance of
the device of FIG. 2A as a function of current. The resistance of
MTJ 204 of FIG. 2A may be based on the whether the magnetization of
the free layer and the fixed layer are in the same direction
(referred to as parallel state) for low impedance, or in opposite
direction (referred to as anti-parallel) for high impedance. FIG.
2B illustrates that is possible to change the magnetization
direction of the free layer based on the current flowing through
the spin-orbital coupling channel 202.
[0049] MTJ 204 may be used for purposes of writing a digital value
that is subsequently read. In this way, MTJ 204 may function as
part of a memory cell. For instance, the resistance of the MTJ 204
may be associated with a digital value. When the MTJ 204 has a high
resistance, then the MTJ 204 may be associated with a first digital
value (e.g., digital high or digital low), and when the MTJ 204 has
a low resistance, then the MTJ 204 may be associated with a second
digital value (e.g., the other of the digital high or digital
low).
[0050] A controller circuit (not illustrated) may output a current
through the spin-orbital coupling channel 202 that sets the MTJ 204
into the parallel state or anti-parallel state. For instance, the
current may flow through terminals 208 and 210. When MTJ 204 is in
the parallel state, the MTJ 204 may be considered as storing the
first digital value, and when MTJ 204 is in the anti-parallel
state, the MTJ 204 may be considered as storing the second digital
value. In this example, a first terminal 208 may be the input into
the spin-orbital coupling channel 202, and a second terminal 210
may be the output of the spin-orbital coupling channel 202. By
setting the MTJ 204 to parallel or anti-parallel, the controller
circuit may effectively write a first or a second digital
value.
[0051] To read the digital value, the controller circuit may output
a current through the MTJ 204 via terminal 212 and measure the
voltage. If the MTJ 204 is in the parallel state, then the voltage
will be at a first voltage value because the resistance of the MTJ
204 will be low, and if the MTJ 204 is in the anti-parallel state,
then the voltage will be a second voltage value because the
resistance of the MTJ 204 will be high. Accordingly, by measuring
the voltage, the controller circuit may determine the resistance of
the MTJ 204, and hence, the digital value stored by the MTJ 204. In
this example, the terminal through which the current flows through
the MTJ 204 is a third terminal 212, meaning that the example
illustrated in FIG. 2A is for a three-terminal device.
[0052] Accordingly, in the example illustrated in FIG. 2A, three
terminals may be needed for writing and reading. This disclosure
describes example techniques to reduce from three terminals for
writing and reading, to using two-terminals for writing and
reading.
[0053] While spin Hall effects may have led to many memory and
logic device concepts and demonstrations, the devices may be for
three-terminal devices. Developing three-terminal devices,
including the MTJ structure may be complicated and may not suitable
for cross-bar memory and 3D memory architecture. For example,
forming such stacks of MTJ structures on top of one another or in a
cross-bar architecture may be complicated and may not result in
consistent structures exhibiting the same properties.
[0054] This disclosure describes examples of two-terminal
spintronic devices. To achieve two-terminal spintronic devices, the
disclosure describes using unidirectional spin Hall
magnetoresistance (USMR) effects. In USMR, the resistance of a
spin-orbital coupling channel, a magnet, and the interface between
the channel and the magnet, is set based on the current through a
spin-orbital coupling channel. However, no additional terminal is
needed to determine the resistance of the magnet. Rather, a current
through the spin-orbital coupling channel can be used to determine
the voltage across the channel, magnet, and interface, and hence
the resistance of the channel, magnet, and interface.
[0055] FIGS. 3A and 3B are conceptual diagrams illustrating
unidirectional spin Hall magnetoresistance (USMR) effects. Spins
are generated at an interface of a non-magnetic layer 302 (e.g., a
topological insulator (TI)) and a magnetic layer 304 (e.g., CoFeB)
when a charge current (j) is applied to the non-magnetic layer 302.
The relative directions of spins to magnetization of either
parallel or anti-parallel result in different resistance states or
levels. For example, as illustrated in FIG. 3A, the spin of
electrons 314 is anti-parallel to the magnetization direction of
magnetic layer 304, which may in some examples correspond to a high
resistance level. In the example of FIG. 3B, the spin of electrons
314 is parallel to the magnetization direction of magnetic layer
304, which may correspond to a low resistance level. A high
resistance level (e.g., greater than or equal to a threshold
resistance) may correspond to a first digital value (e.g., one of a
digital high or a digital low), while a low resistance level (e.g.,
less than a threshold resistance) may correspond to a second
digital value (e.g., the other one of a digital high or a digital
low). In contrast to conventional three-terminal devices that
utilize an MTJ where the resistance is based on the magnetization
direction of a free magnet relative to the magnetization direction
of a fixed magnet (e.g., the fixed magnet acts as a reference),
according to techniques of this disclosure, the resistance is based
on the magnetization direction of a single magnet relative to the
spin direction of electrons in the spin-orbital coupling channel.
In other words, the unidirectional resistance is not based on the
magnetization direction of the magnet relative to a magnetization
direction of another magnet. Said yet another way, the reference
direction may be the direction of the electron spins of the
spin-orbital coupling channel and not the direction of a reference
magnet.
[0056] The USMR originates from the spin accumulation at the FM
(ferromagnetic)/NM (non-magnetic) interface induced by spin Hall
effect. It is unique compared to anisotropic magnetoresistance or
spin Hall magnetoresistance due to its angular symmetry. It may be
one of few (possibly only) type of magnetoresistance in a FM/NM
structure that gives sensitivity to magnetization states that are
opposite to each other. For example, tunneling magnetoresistance
(e.g., such as in an MTJ) may require an additional reference
FM.
[0057] FIGS. 4A and 4B are conceptual diagrams illustrating an
example system 400 for writing to and reading from, respectively, a
two-terminal spintronic device. For instance, FIGS. 4A and 4B
illustrate the combination of spin Hall effect induced switching
and USMR for two-terminal memory device. System 400 includes
two-terminal memory device 401 and controller circuit 420.
[0058] In some examples, two-terminal memory device 401 includes
contacts 422A and 422B (collectively, contacts 422), and structure
410. Structure 410 includes spin-orbital coupling channel (e.g., a
spin hall channel) 402 and magnet 404. Contacts 422A and 422B may
include any conductive material and may be configured to transport
a voltage or current from controller circuit 420 through
spin-orbital coupling channel 402. In other words, contacts 422A
and 422B may function as terminals for two-terminal memory device
401.
[0059] Spin-orbital coupling channel 402 may include non-magnetic
material, such a heavy metal (e.g., Tungsten (W), Tantalum (Ti),
Platinum (Pt)) or their alloys or their multilayers, a topological
insulator (TI), a doped topological insulator, or a combination
therein. In some examples, a topological insulator may include
Bi.sub.2Se.sub.3 or (BiSb).sub.2Te.sub.3.
[0060] Magnet 404 may include a ferromagnetic material (e.g., Fe,
CoFeB). The magnet 404 may not need to be tailored differently from
spin-orbital coupling channel 402 (e.g., magnet 404 and
spin-orbital coupling channel 402 may be the same shape or
different shapes) as the spin-orbital coupling channel 402 since
the effects may be based on the interface 403, or boundary, between
spin-orbital coupling channel 404 and magnet 404. For example, a
spin hall current along the surface of spin-orbital coupling
channel 402 may exert a spin torque along the interface of
spin-orbital coupling channel 402 and magnet 404, which may change
the magnetization state of magnet 410, and hence the resistance of
structure 410. For instance, changing the magnetization of magnet
404 may change the resistance at the interface 403 between magnet
404 and spin-orbital coupling channel 402, such that the resistance
of structure 410 changes.
[0061] In accordance with techniques of this disclosure, controller
circuit 420 may control write operations to, and read operations
from, two-terminal memory device 401. Controller circuit 420 may
include one or more processors, including, one or more
microprocessors, digital signal processors (DSPs), application
specific integrated circuits (ASICs), field programmable gate
arrays (FPGAs), or any other equivalent integrated or discrete
logic circuitry, as well as any combinations of such
components.
[0062] As illustrated in FIG. 4A, the initial magnetization
direction of magnet 404 may be illustrated by a dashed arrow 406A.
Different magnetization directions of magnet 404 may correspond to
different resistance levels (e.g., a high resistance level or a low
resistance level) of structure 410. The resistance level of
structure 410 may be considered to be a high resistance level when
the resistance satisfies (e.g., is greater than or equal to) a
first threshold resistance and may be considered to be a low
resistance level when the resistance does not satisfy (e.g., is
less than) a second threshold resistance. The first threshold
resistance and the second threshold resistance may be the same
resistance value, or may be different resistance values. Each
resistance level may correspond to a different digital value, such
as a digital high (e.g., a "1") or a digital low (e.g., "0"). In
other words, the resistance value may represent, or be indicative
of, a digital value. In some examples, a high resistance level
corresponds to a digital high and a low resistance level
corresponds to digital low. In some examples, a high resistance
level corresponds to a digital low and a low resistance level
corresponds to digital high.
[0063] To write, controller circuit 420 may apply a strong pulse
(e.g., greater than or equal to a threshold current density) across
the two-terminal memory device 401 so that spins of electrons 414
are generated by spin Hall effect in the channel 402 and are
absorbed by the top magnet 404. In other words, controller circuit
420 may apply a write current to spin-orbital coupling channel 402
by applying a strong pulse to two-terminal memory device 401. In
response to controller circuit 420 applying the write current
across two-terminal device 401, the magnetization (illustrated by
the bolded white arrow 406B) then is switched from left to right
(block lines illustrate an example of one possible trajectory 407).
In some examples, a resistance level of spin-orbital coupling
channel 402 and a resistance level of magnet 404 may be constant,
while a resistance level of an interface 403 between channel 402
and magnet 404 may vary based on the magnetization direction of
magnet 404 (e.g., the resistance of interface 403 may be
unidirectional). The resistance level of structure 410 includes the
resistance of channel 402, the resistance of magnet 404, and the
resistance of interface 403. In some examples, controller circuit
420 changes the overall resistance level of structure 410 by
changing the magnetization direction of magnet 404 (e.g., which may
change the resistance at the interface 403 between magnet 404 and
spin-orbital coupling channel 402), which may change the overall
resistance level of structure 410, and hence change the digital
value represented by the magnetization state of magnet 404. Because
the resistance level of structure 410 is unidirectional and is
based on the magnetization direction of magnet 404, the resistance
level structure 410 may be a first resistance level when a
magnetization direction of the magnet 404 is a first direction
(e.g., 0 degrees) and a second resistance level when the
magnetization direction of the magnet 404 is 180 degrees opposite
the first direction (e.g., 180 degrees).
[0064] For instance, when the initial magnetization direction of
magnet 404 causes the resistance level of structure 410 to be a
first resistance level (e.g., greater than a threshold resistance),
such that the magnetization state may represent a first digital
value (e.g., digital high). Responsive to receiving a write
current, the magnetization direction of magnet 404 may change,
which may cause the resistance of structure 410 to change (e.g., by
changing the resistance of the interface 403) from a first
resistance level to a second resistance level, such that the
magnetization state may represent a second digital value (e.g.,
digital low).
[0065] To read, controller circuit 420 may apply a mild current
(e.g., less than the threshold current density), and the controller
circuit 420 may determine the voltage signal across the structure
410. In other words, controller circuit 420 may apply a read
current to spin-orbital coupling channel 402 to determine the
voltage across structure 410 (e.g., the voltage across spin-orbital
coupling channel 402, magnet 404, and the interface between channel
402 and magnet 404). The mild current may be sine wave modulation.
The measured voltage, and hence resistance, indicates the state of
the magnet 404. Said differently, by measuring the voltage across
structure 410, controller circuit 420 may determine the resistance
level of structure 410, and hence the digital value represented by
magnet 404. Applying the read current to channel 402 may enable
controller circuit 420 to determine the magnetization state of
magnet 404 without utilizing a third terminal, such that
two-terminal spintronic device 401 may function as a memory or
logic device with only two terminals.
[0066] FIGS. 5A-5C are conceptual diagrams illustrating examples of
crossbar and 3D architectures for memory or logic devices using
two-terminal spintronic devices. With the two terminals, the
example devices described in this disclosure may be embedded into
crossbar architectures (FIGS. 5A and 5B) or 3D architectures (FIG.
5C). In accordance with some examples, two-terminal devices without
a selector may be added without adding the third set of contact
lines to support read operations (e.g., in contrast to devices with
an MTJ without a selector which may use three sets of contact lines
or terminals). As another example, two terminal devices with a
selector may be added without adding a fourth set of lines to
support read operations (e.g., in contrast to devices with an MTJ
and selector which may use four sets of lines or terminals). In
some examples, a selector includes a switch (e.g., a transistor)
connected to the controller via a contact line or terminal and
enables the controller to selectively read the memory or logic
device by allowing current to flow through the memory or logic
device when the selector is activated (e.g., the switch on) and
preventing current from flowing through the memory or logic device
when the selector is inactive (e.g., the switch is off).
[0067] In other words, according to techniques of this disclosure,
the memory or logic devices may include one less terminal compared
to devices that utilize MTJs. The two-terminal devices may provide
better compatibility with the current peripheral circuitries found
in STT-RAM for write and read operations, and with higher density
as compared to three-terminal designs. Also, given proper thin film
growth processes, the device may be made vertically, which allows
for even higher areal density.
[0068] Electric field and strain effects may also be used to
further assist with switching with the techniques described in this
disclosure. The techniques may keep the device to minimal two
terminals which makes it easy to adopt with the current STT-RAM
technologies. The device can be made with a bilayer sandwich
structure to further ease the fabrication effort.
[0069] Accordingly, this disclosure describes examples of a device
(e.g., memory or logic device) with two-terminal write and read
operations that acts like an MTJ but carries greater switching
efficiency as spin Hall effect is utilized. The two-terminal device
may support magnetic cross-bar and 3D memory architectures.
[0070] FIGS. 5A, 5B, and 5C illustrate example memory (or logic)
devices 501A, 501B, and 501C, respectively (collectively, memory
devices 501). Memory devices 501A and 501B are illustrated in a
crossbar architecture. Each device of memory devices 501A and 501B
includes a first contactor (also referred to as a contact line or
terminal) 522A, a second contactor 522C, and a structure 510A that
includes spin-orbital coupling channel 502A and magnet 504A. In
some examples, as illustrated in FIG. 5A, memory device 501A may
include a third contactor.
[0071] Spin-orbital coupling channel 502 and magnet 504 may be
aligned horizontally (e.g., as illustrated in FIG. 5A) or
vertically (e.g., as illustrated in FIG. 5B). In some examples,
when spin-orbital coupling channel 502A and magnet 504A are aligned
horizontally, an interface 503A between spin-orbital coupling
channel 502A and magnet 504A may define a plane that is
substantially parallel to a plane defined by the interface between
magnet 504A and first contactor 522A. As another example, when
spin-orbital coupling channel 502A and magnet 504A are aligned
vertically, an interface 503A between spin-orbital coupling channel
502A and magnet 504A may define a first plane that is substantially
perpendicular to a second plane defined by the interface between
magnet 504A and first contactor 522AA, a third plane defined by the
interface between spin-orbital channel 502A and first contactor
522A, or both. In some examples, the second plane and the third
plane are substantially planar with one another.
[0072] In some examples, magnet 504A may be not be directly coupled
to any of the contactors or may be directly coupled to a single
contactor. For example, as illustrated in FIG. 5A, magnet 504A may
be coupled to the contactors indirectly (e.g., via magnet 504A). In
some examples, spin-orbital coupling channel 502A and magnet 504A
are each directly coupled to at least one contactor (e.g., first
contactor 522A, second contactor 522B, or both). For example, as
illustrated in FIG. 5A, spin-orbital coupling channel 502A may be
directly coupled to two contactors (e.g., first contactor 522A and
third contactor 522C), and as illustrated in FIG. 5B, spin-orbital
coupling channel 502A and magnet 504A may each be directly coupled
to first contactor 522A and second contactor 522B. In some
examples, the spin-orbital coupling connector is directly coupled
to one contactor (e.g., as illustrated in FIG. 6B).
[0073] FIG. 5C illustrates an example memory device 501C in a 3D
architecture. In some examples, a memory or logic device may
include a plurality of spin-orbital coupling channels and magnets.
For instance, device 501C includes spin-orbital coupling channels
502A-502C (collectively, spin-orbital coupling channels 502) and
magnets 504A-504C (collectively, magnets 504). Structure 510A
includes spin-orbital coupling channel 502A and magnet 504A,
structure 510B includes spin-orbital coupling channel 502B and
magnet 504B, and structure 510C includes spin-orbital coupling
channel 502C and magnet 504C. In the examples of FIGS. 5A-5C, a top
contactor (e.g., contactor 522A) and a bottom contactor (e.g.,
contactor 522B) may sandwich a spin-orbital coupling channel (e.g.,
spin-orbital coupling channel 502A) and a magnet (e.g., magnet
502A). In other words, spin-orbital coupling channel 502A and
magnet 504A may be located between two contactors. In some
instance, a third contact line or via (e.g., contactor 522C) and/or
a selector (e.g., as illustrated in FIGS. 6A-6D) may also be
located between the two contactors. As illustrated in FIG. 5C,
spin-orbital coupling channel 502A and magnet 504A are aligned
horizontally such that an interface 503A between spin-orbital
coupling channel 502A and magnet 504A may define a plane that is
substantially parallel to a plane defined by the interface between
magnet 504A and first contactor 522A. Similarly, interfaces 503B
and 503C may also define planes that are substantially parallel to
planes defined between the respective magnets and contactors.
However, in some examples, such as illustrated in FIG. 7B, a
spin-orbital coupling channel and magnet may be aligned vertically,
such that an interface between the spin-orbital coupling channel
and magnet defines a plane that is substantially perpendicular to a
plane defined by the interface between the magnet and first
contactor and the interface between the spin-orbital channel and
the first contactor.
[0074] As one example, the disclosure describes a device that
includes a spin-orbital coupling channel (e.g., 502A) configured to
receive a write current and a read current, a magnet (e.g., 504A)
coupled to the spin-orbital coupling channel, wherein the magnet
and spin-orbital coupling channel exhibit unidirectional spin Hall
magnetoresistance (USMR) effects, and a controller circuit (e.g.,
controller circuit 420 illustrated in FIGS. 4A-4B). The controller
circuit 420 may be configured to output the write current through
the spin-orbital coupling channel 502A to set a magnetization
direction of the magnet 504A, which may set the resistance of
structure 510 (e.g., by changing the resistance of interface 503A)
to a first resistance level indicative of a first digital value or
a second resistance level indicative of a second digital value, and
output the read current through the spin-orbital coupling channel
502A to determine whether the resistance of structure 510 is at the
first resistance level or the second resistance level, without
outputting a current through the magnet 504A.
[0075] In some examples, the controller circuit is configured to
determine a voltage across structure 510A that includes
spin-orbital coupling channel 502A and the magnet 504A, and
determine whether the resistance of the structure 510A is at the
first resistance level or the second resistance level based on the
voltage across the structure 510A. The write current may be at a
first current density greater than or equal to a threshold current
density for setting a magnetization direction of the magnet, and
the read current may be at a second current density less than the
threshold current density. The magnetization direction includes one
of a first magnetization direction for which the resistance of the
structure 510 is the first resistance level, or a second
magnetization direction for which the resistance of the structure
510 is the second resistance level. The spin-orbital coupling
channel may be formed from a topological insulator or a heavy metal
in some examples.
[0076] The spin-orbital coupling channel 502A may be a first
spin-orbital coupling channel, the magnet 504A may be a first
magnet, the write current may be a first write current, and the
read current may be a first read current. In some examples, such as
the example illustrated in FIG. 5C, the device may also include at
least a second spin-orbital coupling channel 502B configured to
receive a second write current and a second read current, and at
least a second a magnet 504B coupled to the second spin-orbital
coupling channel 502B, wherein the second magnet and second
spin-orbital coupling channel exhibit the USMR effects (e.g.,
unidirectional resistance at the interface 503B). The controller
may be configured to output the second write current through the
second spin-orbital coupling channel 502B to set a resistance of
the structure 510B to the first resistance level indicative of the
first digital value or the second resistance level indicative of
the second digital value, and output the second read current
through the second spin-orbital coupling channel 502B to determine
whether the resistance of the structure 510B is at the first
resistance level or the second resistance level, without outputting
a current through the second magnet. As one example, the first
magnet and first spin-orbital coupling channel may be arranged in a
crossbar configuration with the second magnet and second
spin-orbital coupling channel. As another example, the first magnet
and first spin-orbital coupling channel are arranged on top of the
second magnet and second spin-orbital coupling channel from a 3D
memory architecture.
[0077] FIGS. 6A-6D are conceptual diagrams illustrating examples of
memory cell layouts. Each memory cell of memory cells 601A-601D
(collectively, memory cells 601) includes a bottom contact line
622A and a top contact line 622B. In some examples, a memory cell
includes a third contact line 622C, also referred to as a via. In
some examples, each memory cell includes a spin-orbital coupling
channel 602, magnet 604, and selector 606.
[0078] FIG. 6A illustrates an example memory cell 601A where the
spin-orbital coupling channel 602 is coupled to the bottom contact
line 622A. The magnet 604 is coupled to the spin-orbital coupling
channel 602 at interface 603. A via 622C is also coupled to the
spin-orbital coupling channel 602. A selector 606 is coupled to, or
is located within, the via 622C, and couples to the top contact
line 622B.
[0079] FIG. 6B illustrates an example memory cell 601B where the
spin-orbital coupling channel 602 is coupled to the bottom contact
line 622A. The magnet 604 is coupled to the spin-orbital coupling
channel 602 at interface 603, and the selector line 606 is coupled
to the spin-orbital coupling channel 602. A via 622C is coupled to
the selector line 606 and the top contact line 622B.
[0080] FIG. 6C illustrates an example memory cell 601C where the
selector line 606 is coupled to the bottom contact line 622A, and
the spin-orbital coupling channel 602 is coupled to the top of the
selector line 606. The magnet 604 is coupled to the top of the
spin-orbital coupling channel 602 at interface 603. A via 622C is
coupled to the spin-orbital coupling channel 602 and to the top
contact line 622A.
[0081] FIG. 6D illustrates an example of a vertical memory cell
601D. A selector line 606 is coupled to the bottom contact line
622A. The magnet 604 and spin-orbital coupling channel 602 are
arranged vertically relative to and on top of the selector line
606. The magnet 604 and spin-orbital coupling channel 602 are
coupled to one another at interface 603. A top contact line 622B is
coupled to the magnet 604 and spin-orbital coupling channel 602.
Selector 606 may be directly coupled to bottom contact line 622A,
spin-orbital coupling channel 602, and magnet 604. Spin-orbital
coupling channel 602 may be directly coupled to magnet 604 and top
contact line 622B. Similarly, magnet 604 may also be directly
coupled to top contact line 622B. In some examples, spin-orbital
coupling channel 602 and magnet 604 are aligned vertically, such
that an interface 603 between the spin-orbital coupling channel 602
and magnet 604 defines a plane that is substantially perpendicular
to a plane defined by the interface between the magnet 604 and
bottom contact line 622A and the interface between the spin-orbital
channel 602 and the bottom contact line 622A.
[0082] The examples illustrated in FIGS. 5A-5C and FIGS. 6A-6D are
provided as a few examples and should not be considered limiting.
There may be other configurations using USMR effects for
constructing memory or logic cells and the techniques are not
limited to the examples provided in this disclosure.
[0083] FIGS. 7A and 7B are conceptual diagrams illustrating
examples of cross bar memory architecture. FIG. 7A illustrates an
example of a cross bar memory device 700A that includes a plurality
of vertical memory cells 701AA-701CC (collectively, memory cells
701) such as those illustrated in FIG. 6D. While cross bar memory
device 700A includes three top contact lines and three bottom
contact lines with a total of nine memory cells 701, cross bar
memory device 700A may include any number of top/bottom contact
lines 722 and memory cells 701.
[0084] FIG. 7B illustrates an example of a cross bar memory device
700B that includes a plurality of vertical memory cells
701AAA-701CCC (collectively, memory cells 701) in a 3D
architecture. For instance, there may be stacks of cross bars on
top of one another. While 3D cross bar memory device 700B includes
three levels of memory cells stacked on top of one another (with
each level including three top contact lines and three bottom
contact lines with a total of nine memory cells 701 per level),
cross bar memory device 700B may include any number levels, where
each level may include any number of top/bottom contact lines 722
and memory cells 701.
[0085] In the examples of FIGS. 7A and 7B, each memory cell of
memory cells 701 includes a selector directly coupled to a bottom
contact line, a spin-orbital coupling channel, and a magnet. Each
spin-orbital coupling channel may be directly coupled to a
respective magnet and top contact line. Similarly, each magnet may
be directly coupled to the respective top contact line. As
illustrated in FIGS. 7A and 7B, each interface between a respective
spin-orbital coupling channel and magnet defines a plane that is
substantially perpendicular to a plane defined by an interface
between the magnet and bottom contact line and the interface
between the spin-orbital channel and the bottom contact line.
[0086] Although FIGS. 7A and 7B are illustrated with vertical cells
like those illustrated in FIG. 6D, the techniques are not so
limited. In general, the various example memory cells described in
this disclosure may be useable for cross bar architecture, and the
example of using the vertical cell is provided as one example to
assist with understanding.
[0087] FIG. 8A is a conceptual diagram illustrating an example of
nanowire cell. In some examples, a memory cell may be formed by
nanowire of spin Hall/magnetic material shelled in magnetic/spin
Hall material. In this example, magnetization is then left/right
hand circular representing information stored (e.g., left hand
circular represents a first digital value, and right hand circular
represents a second digital value). For example, FIG. 8A
illustrates a nanowire memory cell 850A that includes a
spin-orbital coupling channel 802 surrounded by magnet 804 and a
nanowire memory cell 850B that includes a spin-orbital coupling
channel 802 surrounding magnet 804. Circular magnetization
configurations may still provide high/low USMR resistance states
since the relative directions between local magnetic moments and
spins may be either parallel or anti-parallel.
[0088] FIG. 8B is a conceptual diagram illustrating an example of
nanowire memory cells in cross bar memory device 800. In some
examples, cells may be made vertical with nanowires to further
improve lateral density. As illustrated in FIG. 8B, memory device
800 includes a plurality of nanowire cells 850A. However, memory
device 800 may include nanowire memory cells 850B in addition to,
or in place of, nanowire memory cells 850A. While memory device 800
include nine memory cells in a single level, memory device 800 may
include any number of levels (e.g., a 3D architecture) with any
number of memory cells per level. Thus, a memory device 800 may
include a plurality of nanowire cells 850 that each act as a memory
device to store digital values (e.g., digital high or digital
low).
[0089] FIGS. 9A an 9B are conceptual diagrams illustrating an
example technique for sensing magnetic nano-particles. For
instance, the example techniques described in this disclosure may
be used to detect proximity of magnetic particles using USMR. When
magnetic particles 904 are absent (e.g., not proximate to the
spin-orbital coupling channel, as illustrated in FIG. 9A), the
spin-orbital coupling channel 902 shows no USMR (e.g., the
resistance across the channel may be relatively low). When magnetic
particles 904 make contact with the spin-orbital coupling channel
902 (e.g., as illustrated in FIG. 9B), the magnetic/spin Hall
interface is formed and USMR is present (e.g., the resistance
increases to a first or a second value) indicating the
magnetization directions of the magnetic particles 904.
[0090] FIG. 9C is a graph illustrating example threshold for
sensing presence of magnetic nano-particles. For example, when
there are no magnetic nanoparticles proximate to the spin-orbital
coupling channel, the resistance is relatively low. Then, as
nanoparticles start to get more proximate (e.g., closer) to the
spin-orbital coupling channel, the resistance increases, and when
it passes a threshold, a controller may determine that magnetic
nanoparticles are proximate to the spin-orbital coupling channel.
As the particle gets closer and closer to the spin-orbital coupling
channel, the resistance increases, and then plateaus after the
particle makes contact with the spin-orbital coupling channel.
[0091] In this disclosure, the large spin orbit coupling in
topological insulators results in helical spin-textured Dirac
surface states that are attractive for topological spintronics.
These states generate an efficient spin-orbit torque on proximal
magnetic moments at room temperature. However, memory or logic spin
devices based upon such switching may require a non-optimal three
terminal geometry, with two terminals for the `writing` current and
one for `reading` the state of the device. An alternative
two-terminal device geometry is now possible by exploiting the
recent discovery of a unidirectional spin Hall magnetoresistance in
heavy metal/ferromagnet bilayers and (e.g., at low temperature) in
magnetically doped topological insulator heterostructures.
[0092] This disclosure describes observation of unidirectional spin
Hall magnetoresistance in a technologically relevant device
geometry that combines a topological insulator with a ferromagnetic
metal (including conventional ferromagnetic metal). The example
devices show a figure-of-merit (magnetoresistance per current
density per total resistance) that is comparable to the highest
reported values in all-metal Ta/Co bilayers.
[0093] The spin Hall effect (SHE) in non-magnetic (NM) heavy metals
originates in their strong spin-orbit coupling (SOC). When a charge
current flows through a NM heavy metal, the SHE yields a spin
accumulation at the interface with a proximal material. If the
latter is a ferromagnetic (FM) layer, the spin accumulation at the
interface can exchange angular momentum with the magnetic moments
and exert a spin-orbit torque (SOT). In certain configurations and
at sufficiently high charge current density, the magnetization in
the FM can be switched. SOT switching is believed to be potentially
faster and more efficient than spin transfer torque (STT) switching
that is typically used in magnetic tunneling junction (MTJ) devices
for memory and logic applications.
[0094] SOT switching devices consist of a current carrying channel
with a proximal nanomagnet whose magnetization determines the
memory or logic state. Conventional SOT switching devices need two
terminals for `writing` the state of the device and an additional
terminal, usually an MTJ on top of the nanomagnet, for `reading`
the magnetization state of the device. Since the stable states of
the nanomagnet are 180-degree-opposite to each other, symmetry
prevents the sensing of the magnetization state using a
conventional two terminal magnetoresistance, such as anisotropic
magnetoresistance (AMR) or spin Hall magnetoresistance (SMR). The
required presence of a third terminal for reading makes such SOT
switching devices more difficult to fabricate and usually less
appealing for memory and logic applications.
[0095] With the recent discovery of unidirectional spin Hall
magnetoresistance (USMR) in NM/FM bilayers, such as Pt/Co and
Ta/Co, the third terminal of SOT switching devices is no longer
necessary. USMR originates from the interactions between the spins
generated at the NM-FM interface by SOC of the NM and the
conduction channels in the FM. The unique feature of USMR is its
symmetry; it is sensitive to two opposite magnetization states.
Therefore, this disclosure describes a two terminal SOT switching
device that relies on USMR: the nanomagnet is switched by a current
through the NM channel, while the state of the magnetization of the
nanomagnet is simply read out using the USMR.
[0096] While much of the mainstream activity in SOT devices has
focused on heavy metals, such as Ta, Pt and W, recent research has
begun to explore the potential of 3D topological insulators (TIs).
These are narrow band gap semiconductors wherein strong SOC and
time-reversal symmetry yield helical spin-textured Dirac surface
states whose spin and momentum are orthogonal. This `spin-momentum
locking` (SML) has been confirmed using direct measurements such as
photoemission, electrical transport, and spin torque ferromagnetic
resonance, as well as indirect means such as spin pumping. It has
also been demonstrated that the spins can exert torques on a FM as
one would expect of SOT in the NM/FM case.
[0097] In comparison to the NM/FM bilayers, where SOT switching and
sensing using USMR have both been confirmed, the observation of
USMR in TI/FM systems is just beginning to emerge. A large USMR was
observed in
Cr.sub.x(Bi.sub.1-ySb.sub.y).sub.2-xTe.sub.3/(Bi.sub.1-ySb.sub.y).sub.2Te-
.sub.3 bilayer structures at very low temperatures. Here, the
Cr-doped layer is a FM TI with a low Curie temperature and the
other layer is a NM TI. For more pragmatic applications, it is
desirable to explore the USMR phenomenon is heterostructures that
interface a TI with a conventional FM of technological relevance.
Here, the experimental observation of USMR in TI/FM
heterostructures, include (Bi,Sb).sub.2Te.sub.3/CoFeB and
Bi.sub.2Se.sub.3/CoFeB bilayers. As illustrated in FIGS. 3A and 3B,
spins are generated due to the SML of the TI when a charge current,
j, is applied in the bilayer. Depending on the relative directions
between the spins and magnetization of FM, spins at the interface
present different conductance when interacting with the conduction
channels in the FM. The USMR in TI/FM systems is similar to that in
NM/FM systems with the different mechanisms of spin generation.
[0098] In experiments described in this disclosure, USMR was
observed at temperatures between approximately 20 K and
approximately 150 K for (Bi,Sb).sub.2Te.sub.3 (BST) and
Bi.sub.2Se.sub.3 (BS). The largest USMR among the experiment
samples is about 2.7 times as large as the best USMR in Ta/Co
sample, in terms of USMR per total resistance per current density,
observed in 6 QL BS and CoFeB of 5 nm bilayer.
[0099] The devices studied are fabricated from BST (t QL)/CoFeB
(5)/MgO (2) and BS (t QL)/CoFeB (5)/MgO (2) thin film stacks (t=6
and 10), grown by molecular beam epitaxy (MBE) and magnetron
sputtering. Hall bars of nominal length 50 .mu.m and width 20 .mu.m
are tested with harmonic measurements under both longitudinal and
transverse resistance setup. The magnetization of CoFeB is
spontaneously in-plane with little perpendicular anisotropy
field.
[0100] FIG. 10A is a conceptual diagram illustrating resistance
measurement setup and definitions of rotation planes. FIG. 10A
shows the definition of the coordinates and rotation planes. Zero
angles are at x+, z+and z+directions for xy 10, zx 14 and zy 12
rotations respectively. The directions for rotation for increasing
angle are indicated by arrows. A 3 Tesla external field is applied
and rotated in the xy, zx and zy device planes while the first
order resistance R.sub..omega. and second order resistance
R.sub.2.omega. are recorded with 2 mA R.M.S. AC current.
[0101] FIGS. 10B and 10C are graphs illustrating resistance as a
function of angle. For example, FIGS. 10B and 10C show the angle
dependencies of R.sub..omega. and R.sub.2.omega., respectively, of
the BST (10 QL)/CoFeB (5 nm)/MgO (2 nm) sample at 150 K. The
R.sub..omega. exhibits typical SMR-like behavior with the
R.sup.x>R.sup.z>R.sup.y. Similar to the behavior seen in all
metallic NM/FM bilayers, the variation of the second order
resistance R.sub.2.omega. with angle is also proportional to the
magnetization projected along the y-direction. The period of xy and
zy rotations are 360 degrees while a flat line is observed in the
zx rotation. The amplitude of R.sub.2.omega. is about 3 m.OMEGA.
with an average current density of 0.667 MA/cm.sup.2.
[0102] Due to Joule heating of the device and the temperature
gradient across the device plane, the anomalous Nernst effect (ANE)
and spin Seebeck effect (SSE) also contribute to the second order
resistance. To carefully separate this contribution (denoted as
R.sub.2.omega..sup..DELTA.T) from the USMR, a series of
measurements were carried out of Hall or transverse second order
resistance with xy-plane rotations under various external field
strengths.
[0103] FIG. 11A is a conceptual diagram illustrating
transverse/Hall resistance measurement setup. The transverse
resistance is measured while the external field is rotated in the
xy-plane. The second order Hall resistance, R.sub.2.omega..sup.H,
contains contributions from ANE/SSE, field-like (FL) SOT and
anti-damping (AD) SOT. The ANE/SSE and AD SOT are proportional to
cos.phi. while the FL SOT is proportional to cos3.phi.+cos.phi.
(ref.sup.29).
[0104] FIGS. 11B and 11C are graphs illustrating resistance as a
function of angle and external fields, respectively. FIG. 11B shows
two examples of R.sub.2.omega..sup..DELTA.T vs. angle with 20 mT
and 3 T external fields, respectively. Since AD SOT and FL SOT
perturb the magnetization and thus contribute to
R.sub.2.omega..sup.H, their effects diminish at larger external
field. FIG. 11B shows that the data measured in a 20 mT field
contain both cos.phi. and cos3.phi. components, while in a 3 T
field, the data exhibit almost no cos3.phi. component. There are
two steps to obtain the R.sub.2.omega..sup..DELTA.T. First, by
fitting the angle dependent dat, the amplitudes of the cos.phi. and
cos3.phi. components can be extracted. The FL SOT can then be
easily determined and separated. This leaves the ANE/SSE and AD
SOT. A plot of the data corresponding to these contributions versus
the reciprocal of total field is shown in FIG. 11C. In FIG. 11C,
B.sub.dem-B.sub.ani is the demagnetization field minus the
perpendicular anisotropic field of the FM layer, which is
determined to be about 1.5 T by separate measurements. Since the
effect of the AD SOT will diminish at infinite field, the intercept
of the fitted line is the contribution of ANE/SSE to the 2.sup.nd
order Hall resistance. Then, the contribution of ANE/SSE to the
longitudinal resistance R.sub.2.omega. can be obtained by scaling
that from the Hall resistance with the relative ratio of device
length to device width. Finally, the USMR is determined once the
ANE/SSE contribution is subtracted from the R.sub.2.omega..
[0105] FIGS. 12A and 12B are graphs illustrating contribution for
total resistance for different examples of structures that exhibit
USMR effects. FIGS. 12A and 12B show the R.sub.2.omega.,
R.sub.2.omega..sup..DELTA.T and R.sub.USMR of BST and BS samples
with 2 mA and 3 mA current, respectively, at various temperatures.
Temperature affects the chemical potential and the relative
contributions to transport from surface and bulk conduction. As a
result, even though the magnetization and resistivity of the CoFeB
layer vary little within the range of temperature in the
experiments, the charge to spin conversion in TIs and the related
USMR are both strongly temperature dependent. The BST/CoFeB sample
gives the highest USMR at 70 K while the R.sub.2.omega. and
R.sub.2.omega..sup..DELTA.T keep increasing with increasing
temperature up to 150 K. The USMR of BS/CoFeB may only be confirmed
within between 50 K and 70 K because of larger noise and magnetic
field dependent signal outside the temperature window. At 70 K, BST
and BS samples show resistance R.sup.z of 733 .PSI. and 489 .PSI.,
and USMR per current density of 1.067 m.PSI./MA/cm.sup.2 and 0.633
m.PSI./MA/cm.sup.2, respectively. The ratios of USMR per current
density to total resistance of the two samples are 1.45
ppm/(MA/cm.sup.2) and 1.30 ppm/(MA/cm.sup.2), respectively. These
values are slightly better than the best result obtained using
Ta/Co bilayers (1.14 ppm/(MA/cm.sup.2) at room
temperature).sup.9.
[0106] FIGS. 13A and 13B are graphs illustrating USMR per current
density per total resistance and sheet USMR per current density,
respectively, as a function of temperature. For example, FIGS. 13A
and 13B show USMR per current density per total resistance
(R.sub.USMR/j/R) and sheet USMR per current density
(.DELTA.R.sub.USMR/j) of all four samples as a function of
temperature. This provides a more meaningful figure-of-merit for
comparisons of USMR across different types of samples. These two
values also show very similar trends for all samples at various
temperatures, except for the comparison between BST6 and BS10 at
70K, in which BST6 is lower than BS10 in terms of R.sub.USMR/j/R
but higher in terms of .DELTA.R.sub.USMR/j. The swap of position is
mostly due to the larger total resistance of BS10 compared to BST6
while they show similar R.sub.USMR/j. The largest R.sub.USMR/j/R
and .DELTA.R.sub.USMR/j are 3.19 ppm/MAcm.sup.2 and 0.95
m.PSI./MAcm.sup.2, respectively, and both observed in BS6 at 150 K.
It is more than as twice large as the best reported Ta/Co case. As
mentioned before, the USMR measurements beyond the temperature
ranges of the plots of each sample show strong noise and
field-dependent signal background as to render the estimations of
USMR unreliable.
[0107] The above demonstrates the presence of USMR in topological
insulator/ferromagnetic layer heterostructures. The USMR was
observable with a much lower current density compared to all
metallic NM/FM bilayers. The ratios of USMR per current density to
total resistance are found to be comparable to the best result
reported so far in Ta/Co bilayers. The observation of USMR in a
TI/FM system is usable to build a two terminal TI-based SOT
switching device. Such a two-terminal topological spintronic
switching device is potentially more efficient compared to MTJs
that use STT switching due to the large SOC of TIs. The observed
USMR may enable the read operation of such a device without having
to build a MTJ structure on top of TI. Such two terminal devices
may be more architecture friendly and easier embed in current STT
magnetic random access memory architectures.
[0108] The following describes method for forming the bilayer
structures to achieve the USMR effects. The Bi.sub.2Se.sub.3 or
(Bi.sub.1-xSb.sub.x).sub.2Te.sub.3 films were grown by MBE on InP
(111)A substrates. The InP (111)A substrate is initially desorbed
at 450.degree. C. in an EPI (Veeco) 930 MBE under high purity (7N)
As.sub.4 supplied by a Knudsen cell until a 2.times.2
reconstruction is visible in reflection high energy electron
diffraction. The substrate is then moved under vacuum to an EPI 620
MBE for the Bi-chalcogenide deposition. Bi.sub.2Se.sub.3 films were
grown from high purity (5N) Bi and Se evaporated from Knudsen cells
at a beam equivalent pressure flux ratio of 1:14. The substrate
temperature was 325.degree. C. (pyrometer reading of 250.degree.
C.) and the growth rate was 0.17 nm/min. The films have a root mean
squared (RMS) roughness of approximately 0.7 nm over a 25
.mu.m.sup.2 area measured by atomic force microscopy (AFM). For
(Bi,Sb).sub.2Te.sub.3 films, the flux ratio of Bi to Sb was 1:3 and
(Bi+Sb):Te is at a flux ratio of approximately 1:12 for a growth
rate of 0.44 nm/min with a RMS roughness of approximately 1.1 nm
over a 25 .mu.m.sup.2 area measured by AFM. These films are grown
at a substrate temperature of 315.degree. C. (240.degree. C.
measured by a pyrometer) using 5N purity Sb and 6N Te from Knudsen
cells. Film thickness is measured by X-ray reflectivity and crystal
quality by high-resolution X-ray diffraction rocking curves of the
(006) crystal plane--with a full width half max (FWHM) of
approximately 0.28 and 0.11 degrees for Bi.sub.2Se.sub.3 and
(Bi,Sb).sub.2Te.sub.3 films respectively.
[0109] The MBE-grown TIs were then sealed in Argon gas and
transported to an ultra-high vacuum (UHV) six-target Shamrock
sputtering system which could achieve a based pressure better than
5.times.10.sup.-8 Torr at room temperature. The thin films were
first gently etched by Argon ion milling. Then the CoFeB layer was
deposited using a Co.sub.20Fe.sub.60B.sub.20 target. An MgO layer
was deposited to serve as a protection layer. The device
fabrication began with a photolithography followed by an ion
milling etching to define the Hall bars. Then the second
photolithography and an e-beam evaporation followed by a liftoff
were performed to make contacts.
[0110] The devices were tested in a Quantum Design PPMS which
provides temperature control, external field and rotation. The AC
current at 10 Hz was supplied by a Keithley 6221 current source. A
Stanford Research SR830 or an EG&G 7265 lock-in amplifier
paired with an EG&G 7260 lock-in amplifier were used to measure
the first and second harmonic voltages, respectively and
simultaneously.
[0111] FIG. 14 is a graph illustrating resistance as a function of
magnetic field for different structures. For instance, FIG. 14
illustrates USMR in heavy metal/FM systems such as Ta/Fe and
Pt/Fe.
[0112] FIGS. 15A-15C are graphs illustrating resistance as a
function of magnetic field for additional structures are different
temperatures. FIGS. 15A-15C illustrates the use of topological
insulators as spin-orbital coupling channel. The surface states of
a topological insulator (TI) exhibit spin-moment locking. If a
charge current is applied across a TI channel, the electrons on
top/bottom surface are also spin-polarized to right/left. Although
the physical process may be different from spin Hall effect, the
phenomena may be essentially equivalent (e.g., a change current in
the channel induces spins on top/bottom surface pointing
right/left). Torques exerted on a FM by Tis with planar Hall effect
measurements have been observed.
[0113] FIGS. 15A-15C illustrate the USMR in TI/FM systems for
different types of bilayer structures. For example, FIG. 15A
illustrates with bilayer structure of (Bi,Sb).sub.2Te.sub.3/CoFeB,
and FIG. 15B illustrates with bilayer structure of
Bi.sub.2Se.sub.3/CoFeB. There may be even larger USMR when the
CoFeB is replaced with magnetic insultor (MI), yttrium iron garnet
(YIG), as illustrated in FIG. 15C.
[0114] FIG. 16 is a graph illustrating sheet USMR comparison. As
illustrated in FIG. 16, the USMR of YIG/BS, in terms of sheet
resistance per current density, is an order of magnitude larger
than the best USMR observed among TI/CFB systems. The USMR may be
as large as 50.times. of that in the Ta(3)/Fe(2.5). Accordingly,
USMR may provide potential of being further improved towards
practical applications. In some examples, the TIs serve the same
role as other heavy metals or spin Hall materials. These material,
including Tis, are referred to, generally, as spin Hall
material/channel and their functions of generating spins as spin
Hall effect in general.
[0115] FIG. 17 is a flowchart illustrating example operations of a
device configured to write to and read from a two-terminal
spintronic device. For purposes of illustration only, the method of
FIG. 17 will be explained with reference to the example system 400
described in FIGS. 4; however, the method may apply to other
examples.
[0116] Controller circuit 420 outputs a write current through a
spin-orbital coupling channel 402 (1702). For example, controller
circuit 420 may output a pulse that has current density that
satisfies (e.g., is greater than) a first threshold current density
through contact 422A. The pulse may generate a spin current by
causing the spin of electrons 414 at the interface of contact 422A
and spin-orbital coupling channel 402 to align in a particular
direction. The spin current at the interface of contact 422A and
spin-orbital coupling channel 402 may cause the resistance
structure 410 to change from a first resistance level (e.g.,
indicative of a digital high) to a second resistance level (e.g.,
indicative of a digital low). For example, the spin current may set
the magnetization direction of magnet 404 from a first
magnetization direction (e.g., parallel) to a second magnetization
direction (e.g., anti-parallel), which may change the resistance of
interface 403, and thus changing the resistance of structure
410.
[0117] Controller circuit 420 outputs a read current through the
spin-orbital coupling channel 402 (1704). For example, controller
circuit 420 may output a pulse that has a current density that does
not satisfy (e.g., is less than) a second threshold current
density. The second threshold current density may be the same or
different than the first threshold current density. Controller
circuit 420 determines whether the resistance of structure 410 is
at a first resistance value or a second resistance value (1706).
For example, controller circuit 420 may determine a voltage across
structure 410 while outputting the read current. The voltage across
structure 410 may be indicative of the resistance of structure 410
(and hence the resistance of interface 403 between spin-orbital
coupling channel 402 and magnet 404). For example, when controller
circuit 420 determines that the voltage across structure 410 is a
first voltage, controller circuit 420 may determine that the
resistance of structure 410 is a first resistance value. Similarly,
when controller circuit 420 determines that the voltage across
structure 410 is a second voltage, controller circuit 420 may
determine that the resistance of structure 410 a second resistance
value.
[0118] Responsive to determining that the resistance of structure
410 is at the first resistance value ("First" branch of 1706),
controller circuit 420 determines that the memory cell corresponds
to a first digital value (1708). The first resistance value
corresponds to a first digital value (e.g., one of a digital high
or a digital low). Thus, in some examples, when controller circuit
420 determines that the resistance value of structure 410
corresponds to a first voltage, controller circuit 420 may
determine that memory cell 401 represents a first digital value
(e.g., one of a "0" or "1").
[0119] Responsive to determining that the resistance of structure
410 is at the second resistance value ("Second" branch of 1706),
controller circuit 420 determines that the memory cell corresponds
to a second digital value (1710). The second resistance value
corresponds to a second digital value (e.g., the other of the
digital high or the digital low). Thus, in some examples, when
controller circuit 420 determines that the resistance value of
structure 410 corresponds to a second voltage, controller circuit
420 may determine that memory cell 401 represents a second digital
value (e.g., the other of the "0" or "1").
[0120] Various embodiments of the invention have been described.
These and other embodiments are within the scope of the following
claims.
* * * * *